Introduction

Unexpected chromatographic problems sometimes occur when an HPLC system that has been used exclusively for reversed phase methods is converted to HILIC operation.

Symptoms may include:

• Unexpected peaks
• Increased baseline noise
• Baseline drift
• Retention changes
• Reproducibility issues
• Column performance degradation

In many cases, the column is not the source of the problem. Instead, contamination accumulated within the HPLC system itself may be introduced into the chromatographic flow path when the mobile phase is changed.

Understanding this phenomenon can prevent misdiagnosis and unnecessary column replacement.

Why Reversed Phase Systems Accumulate Contaminants

During routine reversed phase operation, many compounds are retained not only on the chromatographic column but also throughout the fluidic system.

Potential accumulation sites include:

• Pump seals
• Check valves
• Solvent inlet frits
• Degasser channels
• Mixing chambers
• Injector components
• Tubing surfaces

Over time, small amounts of:

• Sample residues
• Matrix contaminants
• Hydrophobic compounds
• Environmental debris
• Buffer residues
• Unknown impurities

may accumulate within these system components.  These residues are often not noticeable during routine reversed phase operation.

What Happens When Switching to HILIC Methods

HILIC methods typically employ mobile phases that are substantially different from traditional reversed phase solvents.

Common differences include:

• Higher acetonitrile content
• Different additive concentrations
• Changes in pH
• Different organic solvent exposure

These new solvent conditions may dissolve or mobilize contaminants that had previously remained adsorbed within the instrument.  As a result, accumulated residues may begin to elute into the chromatographic system.

Potential Effects on Chromatography

As contaminants are released from the instrument, several chromatographic issues may appear.

Common observations include:

Unexpected Peaks

Unknown contaminants may appear as previously unseen peaks in chromatograms.

These peaks may:

• Vary between injections
• Change in intensity over time
• Gradually disappear after extensive flushing

Baseline Instability

Released contaminants can contribute to:

• Baseline drift
• Increased noise
• Detector instability
• Variable background signal

Retention Changes

Contaminants entering the column can occasionally alter:

• Retention times
• Selectivity
• Peak shape

making troubleshooting more challenging.

Impact on the Column

The chromatographic column is often the first component suspected when performance issues arise.

However, contaminants originating from the instrument can reach the column and contribute to:

• Inlet contamination
• Frit blockage
• Efficiency loss
• Increased backpressure
• Changes in chromatographic behavior

These effects may make it appear that the column has failed when the actual source of the problem is elsewhere in the system.

Why the Problem Is Often Misdiagnosed

The timing can be misleading.

When chromatographers install:

• A new HILIC column
• A new method
• A new mobile phase

and then immediately observe problems, the newly installed column is often blamed.  However, the appearance of problems immediately after switching solvent systems frequently indicates that contaminants are being released from the instrument rather than from the column itself.

Recommended Preparation Before Converting a System

If a system has been dedicated to reversed phase methods for an extended period, consider performing a thorough instrument cleaning before beginning HILIC method development.

Areas to evaluate include:

• Solvent lines
• Pump components
• Check valves
• Injector assemblies
• Mixing chambers
• Tubing
• Reservoir frits

A preventative maintenance procedure can often help remove accumulated residues before they affect a new method.

Troubleshooting Signs of System Contamination

Instrument contamination should be considered when:

• New unidentified peaks appear after switching to HILIC.
• Baseline behavior changes unexpectedly.
• Multiple columns exhibit similar problems.
• Issues occur immediately after changing mobile phase composition.
• Blank injections show contaminant peaks.

In these situations, investigate the instrument before assuming the column is defective.

Best Practices When Transitioning to HILIC

To minimize problems when introducing HILIC methods:

• Thoroughly flush the system before use.
• Inspect solvent reservoirs and inlet frits.
• Replace or clean contaminated components when necessary.
• Run multiple blank gradients before analyzing samples.
• Verify baseline stability before beginning method development.
• Consider preventative maintenance for systems with extensive reversed phase usage history.

These steps can significantly reduce troubleshooting time.

Key Takeaways

• Reversed phase systems can accumulate contaminants in many fluidic components.
• Changing to HILIC mobile phases may dissolve and release previously retained materials.
• Released contaminants can cause unexpected peaks, baseline instability, retention changes, and apparent column problems.
• The chromato graphic column is often not the root cause of issues observed during an RP-to-HILIC transition.
• Preventative maintenance and thorough system flushing can help avoid these problems.
• When unexplained chromatography appears after switching modes, evaluate the instrument before suspecting the column.